Polyacrylonitrile (pan) polymers with low polydispersity index (pdi) and carbon fibers made therefrom

ABSTRACT

A method for synthesizing polyacrylonitrile (PAN) polymer with a narrow molecular weight distribution is disclosed. The preferred PAN polymer has a PDI (Mw/Mn) of about 2 or less. Such PAN polymer is synthesized by controlled/living radical polymerization using a special RAFT (Reversible Addition-Fragmentation Chain Transfer) agent. Also disclosed is a method for producing carbon fibers from PAN polymer with low PDI.

TECHNICAL FIELD

The present disclosure generally relates to the synthesis ofpolyacrylonitrile (PAN) polymers and methods of forming carbon fibersfrom PAN polymers.

BACKGROUND

Because of the properties such as high specific strength and stiffness,high chemical resistance, and low thermal expansion, carbon fiber hasbeen used widely in aerospace, sports, and commercial industries ofautomobile, wind energy, and other energy saving areas. Typically,carbon fibers are made from polyacrylonitrile (PAN)-based polymers.

Free Radical Polymerization

PAN polymers, traditionally, are made by free radical polymerizationmethod. In free radical polymerization, a catalyst or an initiatorinitiates first to form initial free-radical species. These radicalspecies start to react with monomers to create active centers to formfree monomer-radicals. Then the monomer radicals react with othermonomers to propagate the molecular chain to form polymer radicals.

During the polymerization, sometimes, one radical reacts with otherradical to couple and form a long dead chain, as a combinationtermination, while some radical at the end of one chain may attack ahydrogen atom at the second-to-last carbon atom in the second radicalchain to form a dis-proportionation termination. The polymer radical canalso react with another compound, such as a chain transfer agent, toterminate the propagation reaction of the polymer radical, and to form anew radical from chain transfer agent. This newly formed chain transferradical starts its new chain propagation. Thus, the chain transfer agentreduces the length of polymer radical chain grown. If the rate of thistermination is much higher than the rate of propagation, then very smallpolymers with short chain lengths are formed. Therefore, the chaintransfer agent is used to control the molecular length or weight of thepolymer. Because of the different termination mechanisms, the resultingmolecular chains have different lengths or different molecular weights.As such, the molecular weight of polymers has a distribution. Thisdistribution can be defined by its polydispersity index (PDI), asfollows:

${P\; D\; I} = \frac{{Mw}\mspace{14mu} \left( {{Weight}\mspace{14mu} {average}\mspace{14mu} {molecular}\mspace{14mu} {weight}} \right)}{{Mn}\left( {{number}\mspace{14mu} {average}\mspace{14mu} {molecular}\mspace{14mu} {weigh}} \right)}$

Alternatively, PDI may be expressed as follows:

${P\; D\; I} = \frac{{Mz}\left( {Z - {{average}\mspace{14mu} {molecular}\mspace{14mu} {weight}}} \right)}{{Mw}\left( {{weight}\mspace{14mu} {average}\mspace{14mu} {molecular}\mspace{14mu} {weight}} \right)}$

Mw, Mn, Mz are measured by a GPC (gel permeation chromatography) method.Here, Mw is the weight average molecular weight. Mn is the numberaverage molecular weight and Mz is the Z-average molecular weight or thesize average molecular weight.

A high PDI indicates that the polymer has a large molecular weightdistribution, which means the polymer has very high molecular weightspecies or very low molecular weight species, or both. In other words,the polymer is composed of molecular chains that vary greatly inlengths. The presence of too high molecular weight or too smallmolecular weight species will affect the process-ability of the polymerinto fibers by spinning and the resulting fiber properties, especiallythe too small molecular weight species, due to the fact that the smallmolecular weight species are a kind of molecular defect to polymermechanical properties.

PAN polymer prepared by conventional radical polymerization does notallow control over polymerization. The resulting polymer has largemolecular weight distribution. Thus, there is a difficulty for themechanical property development of the fibers spun from such PAN.

SUMMARY

The present disclosure provides a method for synthesizingpolyacrylonitrile (PAN) polymer with a narrow molecular weightdistribution, and a method for producing carbon fiber precursors fromsuch polymer. The preferred PAN polymer has a PDI (Mw/Mn) of about 2 orless. Such PAN polymer is synthesized by controlled/living radicalpolymerization using a special RAFT (Reversible Addition-FragmentationChain Transfer) agent.

Carbon fibers produced from the fiber precursors exhibit good propertiessuch as uniform cross-section, low micro and molecular defects. Suchgood properties are due to the fact that the low-PDI polymer has auniform Mw, and results in low molecular and micro-defects during carbonfiber manufacturing processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Mercury porosimetry graph showing the micro-poredistribution of a freeze-dried PAN coagulated fiber produced from alow-PDI PAN polymer.

FIG. 2 is a micrograph image of the cross section area of a PAN fiberprecursor produced from a low-PDI PAN polymer.

FIG. 3 is a variability chart for the cross section area of the same PANfiber precursor referenced in FIG. 2.

DETAILED DESCRIPTION

One aspect of the present disclosure is related to a mechanism forcontrolling the PAN molecular weight distribution or PDI bycontrolled/living radical polymerization using special RAFT agents. ThePDI (Mw/Mn) is targeted to around 2 or less, preferably PDI (Mw/Mn) of1.2 to 1.9 (or an alternative PDI (Mz/Mw) of 1.2 to 1.7).

Controlled/Living Radical Polymerization

If the chain termination occurs only after all the monomers are consumedduring radical polymerization, this polymerization is called a livingpolymerization. In this polymerization reaction, the propagation cancontinue if more monomer is added to the reaction. As an ideal livingpolymerization, all chains are initiated at the beginning of thereaction and grow at a similar rate. There is no irreversible chaintransfer or termination. If initiation is rapid with respect topropagation, the molecular weight distribution is very narrow and thechains can be extended by further adding monomers into the reaction.However, in a radical polymerization all chains cannot be simultaneouslyactive. Therefore, some reagent is used to control the propagation andits rate by forming a dormant stage. By reversibly de-activating oractivating the propagation, a rapid equilibrium between the active anddormant chains can be achieved to control the chain growth at a similarrate such that the narrow molecular weight distribution can be obtained.This is called “controlled/living radical polymerization”. The chemicalused herein is called RAFT (Reversible Addition/Fragmentation ChainTransfer) agent.

Synthesis of PAN Polymer

The method for making PAN polymers having a narrow molecular weightdistribution is a solution polymerization method that includes:

-   -   a. combining acrylonitrile (AN) monomer with a solvent, one or        more co-monomers, and a RAFT agent (as defined herein) to form a        solution;    -   b. heating the solution to a temperature above room temperature,        i.e. >25° C., for example, 40° C.-85° C.; and    -   c. adding an initiator to the solution to initiate a        polymerization reaction.

After polymerization is completed, unreacted AN monomers are strippedoff, e.g. de-aeration under high vacuum, and the resulting PAN polymersolution is cooled down. At this stage, the PAN polymer is in a solutionor a dope form ready for spinning.

The polymerization of AN monomers is affected by controlled/livingradical polymerization using a RAFT agent which is a thiocarbonylthiocompound having the following structure:

The effectiveness of the RAFT agent is dependent on the substituents Rand Z. The substituents impact the polymerization reaction kinetics andthe degree of structural control. R group is a free radical leavinggroup. It controls the re-initiation polymerization during RAFTpolymerization. And the Z group controls the stability of C═S bondreactivity and influences the rate of radical addition andfragmentation.

The preferred RAFT agents are selected from the group consisting ofthiocarbonylthio compounds having the following structures:

-   -   where Z₁=—CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(n)—CH₃, n=0-20;        -   —C(CH₃)_(m)—COOH, m=1-2; —C(CH₃)_(m)—COOCH₃, m=1-2;        -   —C(CH₃)_(m)—COOC₂H₅, m=1-2,    -   R₁=

-   -   -   R′₁=—CN;

-   -   -    x=0-1;        -   R″₁=H; —CH₃; —(CH₂)_(m)—COOH, m=1-2;        -   R′″₁=H; —CH₃

-   -   where Z₂=

-   -    R=F, Cl, CN, OCH₃;    -   R₂=

-   -   -   R′₂=—CN;

-   -   -    X=0-1;        -    —C(CH₃)_(m)—COOCH₃, m=1-2;        -    —C(CH₃)_(m)—COOC₂H₅, m=1-2;        -   R″₂=H; —CH₃; —(CH₂)_(m)—COOH, m=1-2;        -   R′″₂=H; —CH₃

-   -   where Z₃=—CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(y)—CH₃, y=1-20;        -   R₃=—CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(y)—CFH₃, y=1-20.

Specific examples of RAFT agents having structures I, II, and III aboveare, respectively:

1) Trithiocarbonate: 2-cyano-2-propyl dodecyl trithiocarbonate (CPDTC)

2) Dithiobenzoate: 2-cyano-2-propyl benzodithioate (CPBZ)

3) Thiocarbonyl disulfide: bis-dodecylsufanylthiocarbonyl disulfide(BDSTD)

Suitable solvents for polymerization include: dimethyl sulfoxide (DMSO),dimethyl formamide (DMF), dimethyl acetamide (DMAc), ethylene carbonate(EC), Zinc Chloride (ZnCl₂)/water, and Sodium thiocyanate (NaSCN)/water.

Co-monomers that are suitable for the synthesis of the PAN polymers maybe one or more vinyl-based acids, including: methacrylic acid (MAA),acrylic acid (AA), itaconic acid (ITA), vinyl-based esters, for example,methacrylate (MA), methyl methacrylate (MMA), vinyl acetate (VA), ethylacrylate (EA), butyl acrylate (BA), ethyl methacrylate (EMA), and othervinyl derivatives, for example, vinyl imidazole (VIM), acrylamide (AAm),and diacetone acrylamide (DAAm).

The PAN polymerization can be initiated by an initiator (or catalyst) ofazo-based compound, for example: azo-bisisobutyronitrile (AIBN),azobiscyanovaleric acid (ACVA), and 2,2′-azobis-(2,4-dimethyl)valeronitrile (ABVN), or others, or an organic peroxide, for example,dilauroyl peroxide (LPO), ditert-butyl peroxide (TBPO), diisopropylperoxydicarbonate (IPP), and others.

According to a preferred embodiment, PAN polymerization is carried outbased on the following formulation, % by weight (wt %): >90% AN monomer;<5% co-monomer; <1% initiator; <1% RAFT agent, based on total weight ofthese four components; and sufficient amount of solvent to form asolution for containing 5 wt % to 28 wt % of final PAN polymer,preferably, 15 wt % to 25 wt %.

The controlled/living radical polymerization method enables control overthe polymer architecture. This includes molecular weight, molecularweight distribution (i.e. polydispersity), functionality, andcomposition. The RAFT agents discussed above function as chain transferagents during the controlled/living radical polymerization of ANmonomers into PAN.

The RAFT polymerization mechanism has four reaction steps: initiation,addition-fragmentation, re-initiation and equilibration, as illustratedbelow using, as an example, CPDTC as the RAFT agent. During PANpolymerization, azo-bisisobutyronitrile (AIBN) is used as an initiatorand DMSO as solvent.

A. Initiation by AIBN (Azobisisobutyronitrile)

B. Addition-Fragmentation with CPDTC

C. Re-Initiation

D. Equilibration

The polymerization is initiated by AIBN. It decomposes to form two freeradicals (Equation 1) and then the radicals start to react with ANmonomer to initiate the polymerization (Equation 2). More (AN) reactswith radicals and forms living polymer or polymeric radical Pn*(Equation 3). CPDTC, as a RAFT agent, reacts or adds to Pn* to form aRAFT adduct radical. This RAFT adduct radical can lead a fragmentationreaction in either direction to get the starting species or a newradical and polymeric RAFT-Pn (Equation 4). This is a reversible step.In reaction Equation 5, the newly formed radical re-initiates thepolymer growth to get another living polymer or polymeric radical Pm*.This living polymer Pm* reacts with the polymeric RAFT-Pn to form a RAFTadduct radical intermediate (Equation 6). This intermediate can fragmentin either direction to control the chains having equal opportunities forPn* or Pm* growth and a narrow PDI. The polymerization will end when allthe monomers and co-monomers are consumed.

The molecular weight of the PAN polymers produced by the methoddescribed above may be within the range of 60 to 500 kg/mole, preferably90 to 250 kg/mole, and mostly preferably, 115 to 180 kg/mole, with PDIof about 2 or less. The molecular weight is measured by a ViscotekGPCmax gel permeation chromatography (GPC) system. During thecharacterization, DMF (dimethyl formamide) with 0.02M LiBr is used asmobile phase with 1 ml/min flow rate. And the column temperature is setat 45° C.

Carbon Fiber Fabrication

The above-described low-PDI PAN polymers are suitable for wet spinningand air-gap spinning (or alternately “dry-jet wet spinning”) to makecontinuous carbon fiber precursors (i.e. white fibers). It has beenfound that the low-PDI PAN polymers have good spinning ability; i.e.,the ease of making fibers from such polymers by spinning process. Theresulting fiber precursors produced from such polymers showcross-section uniformity, tenacity >5 g/denier, and initial modulus >125g/denier, per ASTM 2256.

To make PAN white fibers, the PAN polymer solution (i.e. spin “dope”) issubjected to conventional wet spinning and/or air-gap spinning afterremoving air bubbles by vacuum. The spin “dope” may have a polymerconcentration within the range of 5%-28% by weight, preferably, 15 wt %to 25 wt %, based on the total weight of the solution. In wet spinning,the dope is filtered and extruded through holes of a spinneret (made ofmetal) into a liquid coagulation bath for the polymer to form filaments.The spinneret holes determine the desired filament count of the PANfiber (e.g., 3,000 holes for 3K carbon fiber). In air-gap spinning, avertical air gap of 1 to 50 mm, preferably 2 to 15 mm, is providedbetween the spinneret and the coagulating bath. In this spinning method,the polymer solution is filtered and extruded in the air from thespinneret and then extruded filaments are coagulated in a coagulatingbath. A coagulation liquid used in the process is a mixture of a solventand a non-solvent. Water or alcohol is typically used as thenon-solvent. The ratio of solvent and non-solvent and bath temperatureare used to adjust the solidification rate of the extruded nascentfilaments in coagulation.

The spun filaments are then withdrawn from the coagulation bath byrollers through a wash bath to remove excess coagulant and stretched inhot (e.g. 40° C. to 100° C.) water baths to impart molecular orientationto the filaments, as the first step of controlling the fiber diameter.The stretched filaments are then dried, for example, on drying rolls.The drying rolls may be composed of a plurality of rotatable rollsarranged in series and in serpentine configuration, over which thefilaments pass sequentially from roll to roll and under sufficienttension to provide filaments stretch or relaxation on the rolls. Atleast some of the rolls are heated by means of pressurized steam whichis circulated internally or through the rolls, or electrical heatingelementals inside of the rolls. A finishing oil may be applied to thestretched fibers, prior to drying, in order to prevent the filamentsfrom sticking to each other in downstream processes.

As the second step of controlling the fiber diameter, a superstretchfollows the first fiber draw. This superstretch process is performed ata temperature of 100° C. to 185° C., above the glass transitiontemperature of fiber, preferably at 135° C. to 175° C. Such stretchfurther orientates the molecules to filaments. The superstretched fibermay have a diameter of about 0.4 to 1.5 denier, preferably 0.5-1.0denier.

The processing conditions (including the composition of the spinsolution and coagulation bath, the amount of total stretches,temperatures, and filament speeds) are correlated to provide filamentsof a desired structure and denier. Following the superstretch step, thefiber filaments may pass over one or more hot rolls and then wound ontobobbins.

To convert the PAN white fibers into carbon fibers, the PAN fibers aresubjected to oxidation and carbonization.

During the oxidation stage, the PAN fibers are fed under tension throughone or more specialized ovens, into which heated air is fed. Theoxidation oven temperature may range from 200° C. to 300° C., preferably220 to 285° C. The oxidation process combines oxygen molecules from theair with the PAN fiber and causes the polymer chains to startcrosslinking, thereby increasing the fiber density to 1.3 g/cm³ to 1.4g/cm³. In the oxidization process, the tension applied to fiber isgenerally to control the fiber drawn or shrunk at a stretch ratio of 0.8to 1.35, preferably 1.0 to 1.2. When the stretch ratio is 1, there is nostretch. And when the stretch ratio is greater than 1, the appliedtension causes the fiber to be stretched. Such oxidized PAN fiber has aninfusible ladder aromatic molecular structure and it is ready forcarbonization treatment.

Carbonization occurs in an inert (oxygen-free) atmosphere inside one ormore specially designed furnaces. In a preferred embodiment, theoxidized fiber is passed through a pre-carbonization furnace thatsubjects the fiber to a heating temperature of from about 300° C. to900° C., preferably 350 to 750° C., while being exposed to an inert gas,e.g. nitrogen, followed by carbonization by passing the fiber through afurnace heated to a higher temperature of from about 700° C. to 1650°C., preferably 800 to 1450° C., while being exposed to an inert gas.Fiber tensioning should be added throughout the precarbonization andcarbonization processes. In pre-carbonization, the applied fiber tensionis sufficient to control the stretch ratio to be within the range of 0.9to 1.2, preferably 1.0 to 1.15. In the carbonization, the tension usedis sufficient to provide a stretch ratio of 0.9 to 1.05. Carbonizationresults in the crystallization of carbon molecules and consequentlyproduces a finished carbon fiber that has more than 90 percent carboncontent.

Adhesion between the matrix resin and carbon fiber is an importantcriterion in a carbon fiber-reinforced polymer composite. As such,during the manufacture of carbon fiber, surface treatment may beperformed after oxidation and carbonization to enhance this adhesion.

Surface treatment may include pulling the carbonized fiber through anelectrolytic bath containing an electrolyte, such as ammoniumbicarbonate or sodium hypochlorite. The chemicals of the electrolyticbath etch or roughen the surface of the fiber, thereby increasing thesurface area available for interfacial fiber/matrix bonding and addingreactive chemical groups.

Next, the carbon fiber may be subjected to sizing, where a size coating,e.g. epoxy-based coating, is applied onto the fiber. Sizing may becarried out by passing the fiber through a size bath containing a liquidcoating material. Sizing protects the carbon fiber during handling andprocessing into intermediate forms, such as dry fabric and prepreg.Sizing also holds filaments together in individual tows to reduce fuzz,improve processability and increase interfacial shear strength betweenthe fiber and the matrix resin.

Following sizing, the coated carbon fiber is dried and then wound onto abobbin.

Carbon fibers produced from the above-described low-PDI PAN polymershave been found to have the following mechanical properties: tensilestrength of greater than 700 Ksi (4826 MPa) and tensile initial modulusof greater than 35 Msi (241 GPa), per ASTM D4018 test method.

The benefits and properties of the above-described PAN polymer andcarbon fibers produced therefrom will be further illustrated by thefollowing Examples.

EXAMPLES Example 1 Synthesis of PAN Polymers

PAN polymers were prepared according to the formulations for PANpolymerization shown in Tables 1A-1C.

TABLE 1A Formulations for PAN polymerization Form- Form- Form- Form-Components ulation 1 ulation 2 ulation 3 ulation 4 Acrylonitrile (AN)99.30 99.30 99.30 99.30 Itaconic acid (ITA) 0.70 0.70 0.70 0.70 CPBZ0.113% 0.029% BDSTD 0.359%* 0.045%

TABLE 1B Formulations for PAN polymerization Form- Form- Form- Form-Components ulation 5 ulation 6 ulation 7 ulation 8 Acrylonitrile (AN)99.30 99.30 99.00 98.00 Itaconic acid (ITA) 0.70 0.70 1.00 Methacrylicacid 2.00 (MAA) CPDTC 0.019% 0.009% 0.022% 0.033%

TABLE 1C Formulations for PAN polymerization Form- Form- Form- Form-Components ulation 9 ulation 10 ulation 11 ulation 12 Acrylonitrile (AN)96.00 96.00 97.00 99.00 Itaconic acid (ITA) 1.00 1.00 Methacrylic acid2.00 2.00 (MAA) Methacrylate 2.00 2.00 (MA) Vinyl imidazole 2.00 (VIM)CPDTC 0.030% 0.030% 0.025% 0.022%

In the above Tables, CPDTC, CPBZ, BDSTD are RAFT agents, where:

CPDTC=2-cyano-2-propyl dodecyl trithiocarbonate

CPBZ=2-cyano-2-propyl benzodithioate

BDSTD=bis-dodecylsufanylthiocarbonyl disulfide

Note: * Raft agent is used by mole % based on the total amount ofmonomers.

Controlled/living radical PAN polymerization was performed as follows:

Azo-bisisobutyronitrile (AIBN) was used as an initiator/catalyst andDMSO as solvent. The RAFT agents were used as chain transfer agents.During polymerization, the following sequence of steps was carried out:

-   -   a) Metering DMSO from DMSO storage tank to a reactor, then AN        from AN storage tank to the reactor;    -   b) Purging the reactor with nitrogen;    -   c) Preheating the reactor and adding co-monomers and RAFT agent        into reactor at above room temperature (25° C.);    -   d) Heating up the reactor and then adding the initiator/catalyst        at the desired temperature point of 40-85° C.;    -   e) Starting the polymerization for the time of 15-23 hours at        the temperature of 60-80° C.;    -   f) Cooling down to a low temperature (40-50° C.) and discharging        the polymer solution.

Following polymerization, the molecular weights and PDI of the producedPAN polymers were measured and the results are shown in Tables 2A-2C.

Gel Permeation Chromatography (GPC) was used to analyze the resultantPAN polymers for their molecular weights and polydispersity index (PDI).Viscotek GPCmax/SEC Chromatography System with low angle and right anglelight scattering detectors and RI detector was used. Data were collectedand analyzed using Viscotek OMNISEC Version 4.06 software for theabsolute weight-average molecular weight (Mw) and its distributiondetermination.

TABLE 2A Polymer molecular weights and distribution Form- Form- Form-Form- ulation-1 ulation-2 ulation-3 ulation-4 Mn (g/mole) 37101 4061756777 48177 Mw (g/mole) 59179 63362 101143 82538 Mw/Mn 1.595 1.560 1.7811.713 Mz 81747 82742 148522 120826 Mz/Mw 1.381 1.306 1.468 1.464

TABLE 2B Polymer molecular weights and distribution Form- Form- Form-Form- ulation 5 ulation 6 ulation 7 ulation 8 Mn (g/mole) 78945 12877386125 64265 Mw (g/mole) 155568 217778 159746 113551 Mw/Mn 1.971 1.6911.855 1.767 Mz 236895 327687 226813 167536 Mz/Mw 1.523 1.505 1.420 1.475

TABLE 2C Polymer molecular weights and distribution Form- Form- Form-Form- ulation 9 ulation 10 ulation 11 ulation 12 Mn (g/mole) 72193 6671069560 76579 Mw (g/mole) 147459 121290 137019 150027 Mw/Mn 2.043 1.8181.970 1.959 Mz 237764 173392 195056 224951 Mz/Mw 1.612 1.430 1.424 1.499

All PAN polymers produced from Formulations with RAFT agents yielded PANpolymers with PDI (Mw/Mn) of around 2 or less. PAN polymer produced fromFormulation 6 has a higher molecular weight (Mw) of 217778 g/mole with1.69 PDI after adjusting the dose of RAFT agent and solutionconcentration with respect to Formulation 5.

Example 2 Fabrication of White Fibers

PAN polymer produced from Formulation 5, as described in Example 1, wasused to form carbon fiber precursors (or white fibers) by wet spinning.PAN polymers produced from Formulation 12, as described in Example 1,was used to form white fibers by air-gap spinning method with 150 μmspinneret.

Properties of the white fibers were determined as follows.

Cross-Section Analysis

White fiber bundle sample was submerged into acrylic resin and thencured. The cured fiber resin rod is polished on a grounder withdifferent grade sander paper for smooth cross-section. After that, thefiber cross-section is measured under an optical microscopy withimage-analysis system for cross section uniformity.

Porosimetry

For air-gap spinning, fiber sample exiting coagulation bath wasfreeze-dried at −60° C. and the freeze-dried sample was tested by amercury porosimeter for porosity and porous structure analysis.

Tenacity & Modulus

Fiber tenacity and initial modulus were measured per ASTM D2256 method.

TABLE 3 White fiber properties & spinning method Formulation Formulation5 Formulation 12 Dope concentration % 18.8 22.14 Spinning method Wetspinning Air-gap Spinneret size 3K 3K Freeze-dried coagulated fiber —85.75 porosity/% Total draw ratio/time 12.2 10.67 White fiber tenacityg/d 7.00 6.54 White fiber modulus g/d 144.3 161

The PAN polymers based on Formulations 5 and 12 were found to have goodspinning ability. The resultant white fiber precursors from both wet andairgap spinnings also had good tenacity and modulus as can be seen fromTable 3.

FIG. 1 is a Mercury porosimetry graph for the distribution of porediameters in the freeze-dried coagulated fiber. The Y-axis is in the logdifferential intrusion in ml/g or dV/dlog D. V is the volume of mercuryintruded into the pores of the sample. X-axis is of pore diameter inlogarithm. Thus, the figure shows the derivative of intruded volume withrespect to the logarithm of pore diameter. The total volume or voids isthe area under the curve. FIG. 1 shows that the freeze-dried PANcoagulated fiber produced by airgap spinning from low-PDI PAN polymeraccording to Formulation 12 has low micro-pore defects. The micrographimage of FIG. 2 and the variability chart of FIG. 3 show that thelow-PDI white fiber spun by airgap spinning has a uniform cross section.FIG. 3 is the variability chart of the cross section area, showing thedispersion or spread.

Converting White Fibers into Carbon Fibers

The white fiber precursors were oxidized in air within the temperaturerange of 220° C.-285° C., and carbonized in nitrogen within thetemperature range of 350° C.-650° C. (pre-carbonization) and then 800°C.-1300° C.

The tensile strength and tensile modulus of the resulting carbon fiberswere determined and are shown in Table 4.

TABLE 4 Carbonization & carbon fiber properties Formulation Formulation5 Formulation 12 Oxidization temperature 220-285 220-285 (° C.)Pre-carbonization 350-650 350-650 temperature (° C.) Carbonizationtemperature  800-1300  800-1300 (° C.) Fiber tensile strength (ksi) 772 800  (5323 MPa) (5516 MPa) Fiber tensile modulus (Msi)   41.9   43.0 (289 GPa)  (296 GPa) Fiber density (g/cm³)    1.809    1.822

Carbon fiber's tensile strength and initial modulus was determined perASTM D4018. The carbon fiber was first impregnated into an epoxy resinbath and then cured. The cured carbon fiber strand is tested on MTSunder 0.5 in/min crosshead speed for its tensile strength and modulus.

Fiber density was determined by liquid immersion method per ASTM D3800.

What is claimed is:
 1. A method for synthesizing a polyacrylonitrile(PAN) polymer with a narrow molecular weight distribution, the methodcomprising: a) combining acrylonitrile (AN) monomer with a solvent, atleast one co-monomer, and a thiocarbonylthio compound to form asolution; b) heating the solution to a temperature above 25° C.; and c)adding an initiator to the solution to affect polymerization reaction,wherein polymerization is affected by controlled/living radicalpolymerization, in which the thiocarbonylthio compound functions as aReversible Addition/Fragmentation Chain Transfer (RAFT) agent, whereinthe thiocarbonylthio compound is selected from the following structures:

where Z₁ is selected from: —CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(n)—CH₃, n=0-20;—C(CH₃)_(m)—COOH, m=1-2; —C(CH₃)_(m)—COOCH₃, m=1-2; —C(CH₃)_(m)—COOC₂H₅,m=1-2; R₁ is selected from:

R′₁ is selected from: —CN;

 x=0-1; R″₁ is selected from: H; —CH₃; —(CH₂)_(m)—COOH, m=1-2; R′″₁ is Hor —CH₃

where Z₂ is selected from:

R is selected from: F, Cl, CN, OCH₃; R₂ is selected from:

R′₂ is selected from: —CN;

 x=0-1;  —C(CH₃)_(m)—COOCH₃, m=1-2;  —C(CH₃)_(m)—COOC₂H₃, m=1-2; R″₂ isselected from: H; —CH₃; —(CH₂)_(m)—COOH, m=1-2; R′″₂ is H or —CH₃

where Z₃ is selected from: —CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(y)—CH₃, y=1-20; R₃is selected from: —CH₂—(CH₂)₁₀—CH₃; —(CH₂)_(y)—CH₃, y=1-20, wherein thePAN polymer has a polydispersity index (PDI) of about 2 or less.
 2. Themethod of claim 1, wherein the PAN polymer has a molecular weight withinthe range of 60 kg/mole to 500 kg/mole.
 3. The method of claim 2,wherein the PAN polymer has a molecular weight within the range of 115kg/mole to 180 kg/mole.
 4. The method according to claim 1, wherein thePAN polymer has a PDI (Mw/Mn) of 1.2 to 1.9 (or an alternative PDI(Mz/Mw) of 1.2 to 1.7).
 5. The method according to claim 1, wherein thesolvent is selected from the group consisting of: dimethyl sulfoxide(DMSO), dimethyl formamide (DMF), and dimethyl acetamide (DMAc),ethylene carbonate (EC), a mixture of zinc chloride (ZnCl₂) and water,and a mixture of sodium thiocyanate (NaSCN) and water.
 6. The methodaccording to claim 1, wherein the at least one co-monomer is selectedfrom the group consisting of: vinyl-based acids, vinyl-based esters, andvinyl derivatives.
 7. The method of claim 6, wherein the at least oneco-monomer is selected from the group consisting of: methacrylic acid(MAA), acrylic acid (AA), itaconic acid (ITA), methacrylate (MA), methylmethacrylate (MMA), vinyl acetate (VA), ethyl acrylate (EA), butylacrylate (BA), ethyl methacrylate (EMA), vinyl imidazole (VIM),acrylamide (AAm), diacetone acrylamide (DAAm).
 8. The method accordingto claim 1, wherein the initiator is an azo compound or an organicperoxide.
 9. The method according to claim 8, wherein the initiator isselected from the group consisting of: azobisisobutyronitrile (AIBN),azobiscyanovaleric acid (ACVA), 2,2′-azobis-(2,4-Dimethyl) valeronitrile(ABVN), dilauroyl peroxide (LPO), ditertbutul peroxide (TBPO),diisopropyl peroxydicarbonate (IPP).
 10. The method according to claim1, wherein the temperature at step (b) is within the range of 40° C.-85°C.
 11. The method according to claim 1, wherein the thiocarbonylthiocompound is selected from: a) 2-cyano-2-propyl dodecyl trithiocarbonate(CPDTC)

b) 2-cyano-2-propyl benzodithioate (CPBZ)

c) bis-dodecylsufanylthiocarbonyl disulfide (BDSTD)


12. A polyacrylonitrile (PAN) polymer produced by the method of claim 1.13. A method of producing a carbon fiber comprising: forming a polymersolution of the PAN polymer produced according to the method of claim 1;spinning the polymer solution by wet spinning or air-gap spinning toform a PAN fiber precursor; oxidizing the PAN fiber precursor; andcarbonizing the oxidized fiber precursor, wherein the carbon fiber has atensile strength of greater than 700 ksi (or 4826 MPa), and an initialmodulus of greater than 35 msi (or 241 GPa), per ASTM D4018 test method.14. The method of claim 13, wherein the polymer solution of PAN polymerfor spinning has a polymer concentration within the range of 5%-28% byweight based on the total weight of the solution.
 15. The method ofclaim 13, wherein oxidizing is carried out within the temperature rangeof 200° C.-300° C.
 16. The method of claim 13, wherein carbonizingincludes pre-carbonization in an inert gas at a lower first temperaturewithin the range of 300° C.-900° C., followed by carbonization at ahigher second temperature within the temperature range of 700° C.-1650°C., said second temperature being higher than the first temperature.